Verticillium wilt of potato in South Africa

Since the first report of Verticillium wilt of potato in 1950, the disease has been considered to be of minor importance in South Africa. Between 1995 and 2000, however, Verticillium spp. were isolated from 146 samples of symptomatic potato plant material received from 13 of the 14 potato production areas in the country. Of 93 Verticillium isolates that were obtained, 60% were identified as V. dahliae and 8 % V. nigrescens. V. dahliae was present in nine of the regions and V. nigrescens in seven. Unidentified Verticillium species were isolated from six of the regions. Both V. dahliae and V. nigrescens were pathogenic to potato in vivo, with V. dahliae the more virulent of the two species. Ten South African potato cultivars, eight of which have recently been released, were evaluated over two seasons in a greenhouse for resistance to V. dahliae. The cultivars Aviva, BP1, Bravo, Buffelspoort, Caren, Hoevelder and Ropedi were classified as susceptible to Verticillium wilt, whereas Calibra, Dawn and Devlin were rated as very susceptible. No resistance or tolerance was evident. The efficacy of broccoli volatiles on in vitro mycelial growth of Verticillium dahliae, and the effect of incorporation of fresh and dry broccoli residues on the survival of microsclerotia of V. dahliae and infection of potato, were determined in the laboratory and greenhouse. Volatiles emanating from freshly harvested macerated broccoli leaves were inhibitory to mycelial growth of V. dahliae on medium. Fresh and dry residues incorporated into soil artificially infested with V. dahliae, significantly reduced the viability of microsclerotia of the pathogen and the rate of infection of potato plants. Dry residues were more effective than fresh residues in reducing the viability of sclerotia, but suppression of infection was independent of the state of the residues.